The interferometers for determining the position of the measuring stage are utilized for each of the axes. The sources of error inherent in the interferometer gain an increasing influence, as requirements become more stringent with respect to the precision of the measurement made by the interferometer of the measuring stage position in any one of the axes. When interferometric displacement measurements in the range of sub-nanometer accuracy are considered, it must be noted that these measurements are influenced by periodic deviations. The periodic length depends on the laser wavelength used and the structure of the interferometer. Typical periodic lengths are 158 nm and 316 nm. The amplitudes are typically in the range of less than 2 nm. In the case of measurements over larger distances these errors are usually negligible. Interferometer errors arise from errors in the optical alignment and from errors relating to the polarization of the components of the interferometer.
U.S. Pa. No. 6,738,143 discloses a system and a method for the compensation of non-linearites of an interferometer. Herein a plurality of digital positional values are recorded, which are divided into corresponding groups. A first group of digital positional values is processed digitally to generate a plurality of data values. The plurality of data values is then digitally processed to generate at least quasi static, non-linear parameters. A second group of digital positional values are compensated on the basis of the non-linear parameters determined by means of the first group of positional data. This method involves high computation overhead and is thus not conducive to the high throughput of a coordinate measuring machine.
Unpublished German Patent Application DE 102007018115 discloses a method for increasing measuring accuracy in the determination of coordinates of structures on a substrate. Herein the substrate is placed on a stage movable in the X and Y coordinate directions. First, a plurality of images of a structure on the substrate are recorded by means of a two-dimensional detector during the relative movement of a measuring lens in the Z coordinate direction and the simultaneous movement of the stage in the X and Y coordinate directions. The method proposed here requires high precision motor control and is thus difficult to implement. The difficulties encountered in implementation are due to both the mechanics and the feedback control required therefor.
German Patent Application DE 102007017630 A1 discloses a method for increasing measuring accuracy in the determination of coordinates of structures on a substrate. Herein a stage traversable in the X and Y coordinate directions is provided, which is placed in an interferometric optical measuring system. The structure on the substrate is imaged onto at least one detector by a measuring lens with an optical axis aligned in the Z coordinate direction. The structure is imaged with a so-called dual-scan, that is, the images of the structure are recorded once in the Z coordinate direction and once in the reverse Z coordinate direction. The image with the highest definition in each traversing direction is selected for evaluation of the position of the structure from the images recorded for each direction. Systematic errors can thus be eliminated with this procedure. In dual-scan, each position must be measured twice. As a consequence measuring time is increased and the throughput of the coordinate measuring machine is reduced accordingly.
German Patent Specification DE 1963777 C1 discloses a method and an apparatus for error correction of an interferometer. Herein a repeated calibration measurement for a constant calibration distance divided up into two distance segments is carried out, wherein the one distance segment is measured using an interferometer and the other segment is measured with an additional distance measuring system that does not present an error which is periodic with respect to the distance measured. Prior to each calibration measurement the distance segment measured by the interferometer is modified marginally. The second distance segment is modified accordingly in the opposite direction. The sum of the distance segment modifications of the partial segment must correspond to at least one optical path length modification of the interferometer by the measuring wave length. Using the various calibration measuring results for the constant calibration distance the periodic error components are separated and a wavelength dependent error curve is determined in order to correct any subsequent interferometer measurements. The teachings disclosed within the scope of this protection right are not suitable for maintaining high throughput of the coordinate measuring machine.
U.S. Pat. No. 6,137,574 discloses a system and a method for characterizing and correcting cyclical errors in the distance measurement of interferometers. The document refers to additional aids which can be placed in the beam path of the interferometer to correct the errors attributable to the interferometer.
The article by Norman Bobrov: “Residual errors in laser interferometry from air turbulence and non-linearity” in: Applied Optics, Vol. 26, No. 13, pp. 2676 to 2682, 1987 also discloses the problem of interferometers used for position determination exhibiting a cyclical error. The inventive method or inventive apparatus is not disclosed with which the errors of the interferometers used to determine the position of a measuring stage can be corrected.
German Patent Application DE 102005052758 discloses a substrate holder apparatus for application in a position measuring device (coordinate measuring machine). This document also addresses the problem of interferometer error in the determination of the position of the measuring stage in the coordinate measuring machine. The method disclosed in the invention is not dealt with here.
U.S. Patent Application 2002/0186376 A1 discloses an etalon for adjusting the effective length of the etalon.